1. Field of the Invention
The present invention relates to a plasma torch furnace for the pyrolysis of carbonaceous and other spent potliner Wastes, to the conversion and recycling of hazardous wastes to useful or non-hazardous substances, and to the recovery and conservation of heat energy for the production of electricity and for process operations.
2. Background Information
The Hall-Heroult process for the production of metallic aluminum dates from the 19th century. Many refinements to the process have been made, but the basic Soderberg or Pre-bake configurations using Hall-Heroult cells remain the most common processes for aluminum production throughout the world. In these processes the bottom and internal walls of a cathode of a Hall aluminum pot are formed with a liner of carbon blocks joined by a conductive carbonaceous binder and wrapped with refractory firebricks and insulating bricks, the resulting combination being referred to as "potliner". The insulating bricks and fire bricks are composed of materials such as silica and alumina.
During the production of aluminum, the aluminum reduction pot is filled with a bath of alumina and molten salts. Over time, the migration of bath salts into the potliner results in the deterioration and eventual failure of the aluminum cell cathode. During its three to seven year life, a cathodic potliner may absorb its own weight in bath materials. The failed potliner material is referred to as "spent potliner, or "SPL".
When an aluminum reduction pot is taken out of service, the SPL is cooled and fractured to facilitate subsequent handling and disposal. The fractured SPL is a non-homogeneous material which contains carbon, silica or alumina from the insulating brick and fire bricks, aluminum, significant quantities of sodium salts, aluminum salts and oxides, fluoride salts, and traces of cyanides.
A typical cathode waste SPL composition might contain, for example:
______________________________________ Component Weight % ______________________________________ Carbon (as C) 33.1 Fluoride (as F) 15.7 Aluminum (as Al) 15.1 Sodium (as Na) 14.2 Silica (As SiO.sub.2) 2.7 Calcium (as Ca) 1.8 Cyanide (as CN) 0.3 Sulfur (as S) 0.1 Subtotal 83.0 Oxygen and Other Trace Materials 17.0 Total 100.00 ______________________________________
On the average, a large aluminum smelter with a production capacity of 175,000 tons of aluminum per year will produce about 6,000 to 12,000 tons of SPL per year. The quantity of SPL generated annually in the United States alone has in recent years exceeded approximately 230,000 tons per year, while word-wide production of SPL is several times this quantity. The estimate for SPL stored in recoverable storage in 1991, in the U.S.A. alone, exceeded some 1.9 million tons, most of which is awaiting proper disposal.
Because of its cyanide content, its high concentration of leachable fluoride compounds, and the high volumes of SPL produced, SPL represents a significant environmental hazard and a major burden for aluminum producers, who remain ultimately liable for its proper disposition. SPL has long been listed as a hazardous waste by the U.S. Federal and state environmental authorities. Current Federal and most state regulations require that SPL ultimately be treated to explicity remove the toxic cyanide, high concentration of leachable fluoride compounds, and other characteristics which caused it to be listed as a hazardous waste before it can be placed in a landfill disposal site. However, pending the promulgation of a best practicable technologies, U.S. authorities have allowed SPL to be stored at qualified facilities until suitable methods of treatment and disposal are found.
U.S. courts have decreed that the U.S. EPA must promulgate specific regulations governing the landfill disposition of SPL by early 1993. The U.S. EPA has indicated that it will establish performance-based standards and encourage recycling and reuse of SPL materials, rather than treatment processes which take as their input the "end of the pipe" flow of wastes from the production process.
Many different approaches have been tried over the years to convert SPL to non-hazardous materials. About ten basic treatment processes for SPL are known, with several having been tried, but none having proven fully satisfactory. Most have applied either some form of combustion or chemical treatment in their efforts to convert SPL to non-hazardous materials. Incineration has had limited success largely because the combustion process has itself yielded significant concentrations of hazardous by-products, albeit different products, and such products are often of equal or greater volume than the starting SPL. Chemical processes have suffered a similar fate, replacing initial SPL constituents with compounds which are relatively less toxic, but which are still above the hazardous listing levels established by environmental authorities, with the residues being of comparable volume to the input.
Efforts have been made to decontaminate SPL by kiln calcination. However, such systems have been found to exhibit extreme operating difficulties in subsequent treatment of ash for fluoride, or in adding sand and limestone to produce a "class A" landfill by the addition of sand and limestone.
Management of SPL by the chemical extraction and recovery of fluorides has been the subject of U.S. Pat. No. 2,858,198. Also, a number of attempts have been made at incinerating SPL by fluidized bed combustion, e.g., U.S. Pat. Nos. 4,763,585 and 4,993,323. The latter patent provides a pyrosulpholysis process by which SPL is pyrolized in a high temperature fluidized bed while converting the fluoride to HF for subsequent recovery in an alumina dry scrubber. To-date, however, this process has reportedly produced nuggets which may still contain unconverted hazardous fluoride material which may be leached into the environment when subject to fracturing, such as typically occurs during bulk transportation of such brittle material to storage sites. Also, slag magma has tended to plug the fluidized bed during tests. A recent paper by Comalco Aluminum Ltd., modified this method by the use of a torroidal fluidized bed, but the paper still teaches and requires the complex treatment of wastes, such as the crushing of SPL to 1 millimeter granules before further treatment. Furthermore, all of these complex fluidized bed systems result in small reduction of the net volume of residual waste, demand a large investment in equipment and require significant plant space.
It is therefore seen that a SPL treatment process is needed which more completely eliminates the hazardous material in the SPL, while reducing the volume of wastes and/or recycling or converting the residuals to benign and useful materials. An ideal process would also be energy efficient, would minimize the handling and transport of hazardous SPL material, and would produce or recycle products having economic value. In keeping with the philosophy recently expressed by the U.S. EPA and espoused by many state environmental authorities, the ideal process would be closely integrated with the process involved in the production of the waste, thereby reducing the net amount of waste emerging from the production operation. Such a process should also be relatively compact to permit close integration of recovery and recycling processes within the aluminum production process.
The aluminum production process has several basic features which make it amenable to a more ideal SPL disposal process. For example, all aluminum smelting plants use large amounts of direct current electric power. Modern aluminum smelters operate at 200-600 mw of A.C. electric power which is converted in a rectifier yard to D.C. electric power for use in the aluminum reduction pots. Therefore, an ideal SPL treatment process at an aluminum production site might teach the use of electricity as its primary energy source. Energy might also be recovered from the use of this high quality electricity energy source, to provide, for example, process heat to ancillary production processes such as paste plant operations. Moreover, as noted above, SPL has an average carbon content of about 33%, resulting in a potential energy yield from SPL of 9 million BTU's per ton. The ideal process might also extract energy from this carbon source.
The Hall-Heroult aluminum reduction process often requires that fluoride be added to maintain the desired conditions of the salt bath in the aluminum reduction cell. Modern aluminum reduction plants usually have alumina counter-flow dry scrubbers so that fluoride gases can be adsorbed on the alumina before it is added to the cell. As noted above, fluoride ions represent almost 16% of SPL, thereby making the estimated value of fluorides which are potentially recoverable from 1991 SPL production about $43 million. Therefore, an ideal treatment process might teach the recovery and recycling of fluorides from SPL to the aluminum production process by the use of the existing alumina counter-flow dry scrubbers.
Finally, the removal of SPL requires its replacement with both carbon and with costly new refractory and fire brick linings. An ideal process for SPL treatment might teach the recovery and reuse of the refractory constituents in the SPL to provide refractory and fire brick linings for a variety of uses.
A technology which may be adaptable to the purpose of treating SPL is the use of heat supplied by a plasma arc torch. Plasma torch technology was substantially advanced through the 1960's when new plasma arc generators were developed to simulate the very high temperature conditions experienced by space vehicles re-entering the Earth's atmosphere. Unlike a combustion burner flame, a plasma arc torch can be operated in the absence of oxygen. A plasma arc is created by the electrical dissociation and ionization of a working gas to establish temperatures at the plasma arc centerline as high as 50,000.degree. K. Commercially available plasma torches can develop flame temperatures in a furnace or work piece as high as 8000.degree. C., or higher for sustained periods at the point of application and are available in sizes from about 100 Kw to over 6 Mw in output power.
A typical plasma torch consists of an elongated tube through which the working gas is passed, with an electrode centered coaxially within the tube. In one type of such torch, a high direct current voltage is applied across the gap between the end of the center electrode as an anode and an external electrode acting as a cathode. The external electrode might be the materials undergoing treatment, or it might be the container surface itself. The current flowing through the gas in the gap between the anode and cathode causes the formation of an arc of high temperature electromagnetic wave energy that is comprised of ionized gas molecules. Any gas or mixture of gases, including air, can be passed through the plasma torch, but nitrogen is the preferred gas for many applications because is has been found to permit a high energy transfer rate and is relatively inexpensive.
Plasma torch systems have been applied to a variety of processes, including some uses for the destruction or conversion of waste and hazardous materials. Examples include the destruction of liquid toxic wastes, and more recently, the pyrolysis of organic and inorganic materials and the recovery of aluminum metals from aluminum dross. U.S. Pat. No. 4,479,433 discloses a method and apparatus for the thermal decomposition of stable compounds. U.S. Pat. Nos. 4,438,706 and 4,509,434 disclose a procedure and equipment for destroying waste material. U.S. Pat. No. 4,644,877 discloses a method and apparatus for the pyrolytic destruction of toxic and hazardous waste materials. However, prior to the present invention, no process has been taught or suggested for the application of a plasma torch to the disposal and decontamination of SPL. Furthermore, no process has been taught or previously suggested for the application of a plasma torch for the disposal and decontamination of SPL in a configuration which integrates a plasma torch processing system into an aluminum production process at an aluminum production site, to thereby reduce the net hazardous SPL waste and the non-hazardous waste created by the aluminum production process, and to improve the economic efficiency of that process, in the regulated environment, by the recovery and recycling of valuable materials from SPL waste.